1. Field of the Invention
The present invention relates to an air-fuel ratio feedback control system in an internal combustion engine having a single air-fuel ratio sensor downstream of or within a three-way reducing and oxidizing catalyst converter within an exhaust gas passage.
2. Description of the Related Art
As known air-fuel ratio feedback control systems using air-fuel ratio sensors (O.sub.2 sensors), there exist a single O.sub.2 sensor system having a single O.sub.2 sensor and a double O.sub.2 sensor system having two O.sub.2 sensors one upstream and one downstream of the catalyst converter. Note, in a single O.sub.2 sensor system, the O.sub.2 sensor is disposed either upstream or downstream of the catalyst converter.
In a single O.sub.2 sensor system having an O.sub.2 sensor upstream of the catalyst converter, the O.sub.2 sensor is disposed in an exhaust gas passage near to a combustion chamber, i.e., near the concentration portion of an exhaust manifold, upstream of the catalyst converter. In this system, however, the output characteristics of the O.sub.2 sensor are directly affected by the non-uniformity or non-equilibrium of the exhaust gas. For example, when the air-fuel ratio actually indicates a rich state, but oxygen is still present, the output characteristics of the O.sub.2 sensor are fluctuated. Also, in an internal combustion engine having a plurality of cylinders, the output characteristics of the O.sub.2 sensor are also directly affected by individual differences between the cylinders, and accordingly, it is impossible to detect the mean air-fuel ratio within the entire engine, and thus the accuracy of the controlled air-fuel ratio is low.
On the other hand, in a single O.sub.2 sensor system having an O.sub.2 sensor downstream of the catalyst converter, the non-uniformity or non-equilibrium of the detected exhaust gas can be eliminated, and the mean air-fuel ratio within the entire engine can be detected. In this system, however, due to the long distance between the O.sub.2 sensor and the exhaust valves, and because the capacity and cleaning efficiency of the catalyst converter depends upon its O.sub.2 storage effect, the response characteristics of the O.sub.2 sensor are lowered, thus reducing the response characteristics of an air-fuel ratio feedback control system. As a result, the efficiency of the catalyst converter cannot be sufficiently exhibited, thus increasing HC, CO, and NO.sub.x emissions.
Also, in the above-mentioned double O.sub.2 sensor system, an air-fuel ratio feedback control operation is carried out by the downstream O.sub.2 sensor in addition to an air-fuel ratio feedback control operation by the upstream O.sub.2 sensor. For example, the mean air-fuel ratio is detected by the downstream O.sub.2 sensor to reflect an air-fuel ratio feedback parameter such as skip amounts, and the air-fuel ratio feedback control for the entire system is carried out by the output of the upstream O.sub.2 sensor and the air-fuel ratio feedback control parameter (see U.S. Pat. No. 4,693,076). Therefore, even if the output characteristics of the downstream O.sub.2 sensor are not stable, good emission characteristics are obtained. In this double O.sub.2 sensor system, however, two O.sub.2 sensors are required, thus increasing the manufacturing cost, and further, when the frequency of the air-fuel ratio feedback control by the upstream O.sub.2 sensor is increased by the aging of the parts of the engine or the like, the efficiency of the catalyst converter is lowered.
In view of the foregoing, the present inventor has already suggested a single O.sub.2 sensor system having a downstream O.sub.2 sensor in which a self-oscillating term AF.sub.s having a definite amplitude and a definite period is generated, and a mean value AF.sub.c of the self-oscillating term AF.sub.s is changed in accordance with the output of the downstream O.sub.2 sensor, to thereby exhibit a full efficiency of the catalyst converter (see Japanese Unexamined Patent Publication (Kokai) No. 64-66441 published on Mar. 31, 1989). This single O.sub.2 sensor system, however, does not provide a sufficient reduction of HC, CO, and NO.sub.x emissions when the O.sub.2 storage effect of the catalyst converter is not fully exhibited. Namely, when the deviation of the air-fuel ratio upstream of the catalyst converter is far from the stoichiometric air-fuel ratio, and this deviation of the air-fuel ratio continues for a very long time, it is impossible to monitor the O.sub.2 storage effect of the catalyst converter. As a result, the precision of the controlled air-fuel ratio is reduced and the HC, CO, and NO.sub.x emissions increased.